Low cost bronze powder for high performance bearings

ABSTRACT

The present invention provides a new process for making a low cost powder for the manufacture of high performance bearings. Improved powders and sintered parts (e.g., bearings) are also provided. The powders of the present invention are formed by blending iron powder with fine cuprous oxide powder and elemental tin powder. The blended powders are then thermally treated under a reducing atmosphere to form a sintered cake, which is milled to a powder.

BACKGROUND OF THE INVENTION

Self lubricated bearings are typically produced from premixed 90 wt. % copper+10 wt. % tin powders+a lubricant. The premix is typically pressed into bearings in a die at pressures ranging from 22,000 lb/in² to 44,000 lb/in² to green densities in 6.0 g/cm³ to 6.3 g/cm³ range. The lubricant helps minimize friction between the die and the bearing during pressing and subsequent ejection from the die, but must be removed in subsequent processing. The green bearings are typically sintered in a three zone belt conveyor furnace. The first zone, called preheat zone, is set at a temperature (typically 1000° F. to 1200° F.) required to burn the lubricant off. The second zone, called high heat zone, is set at a temperature (typically 1500° F. to 1560° F.) necessary to alloy the copper with the tin to form a bronze. The belt speed is adjusted to provide sufficient time at the sintering temperature to ensure homogeneous alloy formation. The third zone, called cooling zone, is used to cool the bearings to room temperature. All three zones have a protective atmosphere used to prevent the bearings from getting oxidized. The bearings undergo Dimensional Change (DC) during sintering, and are typically repressed in a die (sized) to meet the tight dimensional tolerances required with respect to the mating shafts. The dimensional change during sintering is an important property. It must be controlled closely so that the bearings fit easily into the sizing dies, but at the same time do not undergo excessive deformation during sizing, which can lead to cracking. The density of the sized bearings is controlled to provide defined volume of porosity. The sized bearings are then immersed in oil under vacuum. The oil gets impregnated into the porosity by capillary action.

The bearings are used in motors of varied sizes. During use when the motor is turned on, the bearing warms up causing the oil to ooze out of the pores and lubricate the mating surfaces. The reverse occurs when the motor is turned off. As the bearing starts to cool the oil goes back into the pores by capillary action. Best capillary action is achieved when the pores are fine. Lubrication of the mating surfaces lowers the coefficient of friction between the shaft and the bearing, which in turn lowers the wear rate of the bearing. Lower coefficient of friction also lowers the peak and steady state operating temperatures. Lower temperatures are desirable from the standpoint of maintaining the integrity of the oil as well as improving the life of the motor. Another important property of the bearings is the Radial Compressive (Crush) Strength (RCS). Higher strength leads to higher load bearing capacity of the bearing.

To reduce the cost of the bearings, the powder metal industry has used iron powder to replace some of the copper and tin. Such materials are called diluted bronzes. While diluted bronze bearings work satisfactorily in some applications, they typically have higher coefficients of friction and higher peak and steady state operating temperatures than the standard bronze bearings. They also tend to create more noise during operation compared to the standard bronze bearings.

SUMMARY OF THE INVENTION

The present invention provides a new process for making a low cost powder for the manufacture of high performance bearings. Improved powders and sintered parts (e.g., bearings) are also provided.

The powders of the present invention are formed by blending iron powder with fine cuprous oxide powder and elemental tin powder. The blended powders are then thermally treated under a reducing atmosphere to form a sintered cake, which is milled to a powder.

For a better understanding of the present invention together with other and further embodiments, reference is made to the following description taken in conjunction with the examples and figures, the scope of which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

Preferred embodiments of the invention have been chosen for purposes of illustration and description, but are not intended in any way to restrict the scope of the invention. The preferred embodiments of certain aspects of the invention are shown in the accompanying figures, wherein:

FIG. 1 shows a photomicrograph of Powder A, an embodiment of the invention.

FIG. 2 shows a photomicrograph of Powder D, an additional embodiment of the invention.

FIG. 3 shows a photomicrograph of Powder E, yet another embodiment of the invention.

FIG. 4 shows a photomicrograph of a bearing made from Powder A.

FIG. 5 shows a photomicrograph of a bearing made from Powder D.

FIG. 6 shows a photomicrograph of a bearing made from Powder E.

FIG. 7 shows a photomicrograph of a bearing made from the diluted bronze powder F.

FIG. 8 shows a photomicrograph of a bearing made from the diluted bronze powder G.

FIG. 9 shows a photomicrograph of a bearing made from the standard premixed bronze powder.

FIG. 10 compares the DC of sintered bearings of this invention using sponge iron powder with the standard premixed bronze bearings and the diluted bronze bearings, made with the same iron powder, as a function of the sintering temperature.

FIG. 11 compares the RCS of sintered bearings of this invention using sponge iron powder with the standard premixed bronze bearings and diluted bronze bearings, made with the same iron powder, as a function of the sintering temperature.

FIG. 12 shows a photomicrograph of a bearing made from powder H.

FIG. 13 shows a photomicrograph of a bearing made from powder I.

FIG. 14 shows a photomicrograph of a bearing made from powder J.

FIG. 15 shows a photomicrograph of a bearing made from powder K.

FIG. 16 compares the DC of sintered bearings of this invention using atomized iron powder with the diluted bronze bearings, made with the same iron powder, as a function of the sintering temperature.

FIG. 17 compares the RCS of sintered bearings of this invention using atomized iron powder with the diluted bronze bearings, made with the same iron powder, as a function of the sintering temperature.

FIG. 18 shows a photomicrograph of the copper coated iron powder.

FIG. 19 shows a photomicrograph of a bearing made from powder L.

FIG. 20 compares the DC of sintered bearings made from powders L and E, which have similar iron, copper and tin contents, with the standard premixed bronze bearings as a function of sintering temperature.

FIG. 21 compares the RCS of sintered bearings made from powders L and E with the standard premixed bronze bearings as a function of sintering temperature

DETAILED DESCRIPTION OF THE INVENTION

In preparing the preferred embodiments of the present invention, various alternatives may be used to facilitate the objectives of the invention. These embodiments are presented to aid in an understanding of the invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents that may become obvious to those of ordinary skill upon a reading of the present disclosure are included within the spirit and scope of the present invention.

This disclosure is not a primer on the manufacture of bronze powders. Basic concepts known to those skilled in the art have not been set forth in detail.

This invention relates to a new process for making a low cost powder for the manufacture of high performance bearings. Powder cost is lowered by replacing some of the bronze powder with iron powder. However, unlike bearings made from diluted bronze powders of the prior art, the properties of the bearings made from the powder of the instant invention are superior to the conventional bronze bearings, as well as conventional diluted bronze bearings, as shown by the examples.

The process of the invention involves blending an iron powder with fine cuprous oxide and elemental tin powders. Any type of iron powder such as sponge, atomized, electrolytic, etc. can be used; however, sponge iron powder is preferred. Preferred particle size fraction of the iron powder is −150 mesh (<106 μm), although −325 mesh (<44 μm) to −60 mesh (<250 μm) fractions can be used. The cuprous oxide and tin powders should be as fine as possible. Preferred median particles size (D₅₀) of cuprous oxide powder is 10-25 μm, although powders with D₅₀ of up to 50 μm can be used. Preferred D₅₀ of tin powder is 3-12 μm; however powders with D₅₀ up to 20 μm can be used.

The blended powders are thermally treated in a furnace at a temperature ranging from 800° F. to 1600° F., the preferred temperature being about 1300° F. A reducing atmosphere, such as pure hydrogen or nitrogen/hydrogen mixture, is maintained in the furnace. The cuprous oxide is reduced to copper, the tin melts and alloys with the copper to form bronze. The liquid tin also alloys with the iron and helps diffusion bond the bronze to the iron powder. The powders form a sintered cake, which is milled and screened to the desired particle size fraction. Preferred particle size fraction of the milled powder is −60 mesh although size fractions up to −20 mesh can be used. The preferred copper content in the final powder is in the range of 20-50 wt. %, although it can be in the 10-70 wt. % range. The preferred tin content in the final powder is in the 2-5 wt. % range, although it can be in the 1-7 wt. % range.

The powders produced as described above are blended with 0.25 wt. % to 1.00 wt. % powdered lubricant, typically a stearate or wax. The blended powders are pressed into bearings to the desired density. The strength of the bearing in the pressed (green) condition is important for the subsequent handling in the manufacturing process. Green strength is measured on a standard bar according to the Metal Powder Industries Federation (MPIF) standard procedure. The powders of this invention had green strength comparable to or exceeding the green strength of standard premixed and diluted bronze powders.

The bearings are then sintered at an appropriate temperature to achieve the desired sintered density. It is important to avoid excessive DC. The powders of this invention showed remarkably low DC during sintering over a fairly wide temperature range. The standard bronze powders show a considerable variation in DC over the 1500° F. to 1560° F. temperature range used in the industry. Such temperatures are necessary to obtain sufficient RCS discussed above. It was surprising to find that the bearings made from the powders of this invention can be sintered at about 1400° F., which is substantially lower than the standard premixed bronze bearings. The RCS of the inventive bearings are significantly higher than the standard bronze bearings sintered at their normal sintering temperatures. Lower sintering temperature is desirable from the standpoint of energy savings; energy cost is a major factor in the total cost of manufacturing bearings.

The sintered bearings are sized or repressed by putting them back in a die. The low DC of the inventive bearings is a definite advantage as minimal deformation is required to size the bearings to the desired dimensional tolerances. As mentioned above, excessive deformation can cause the bearings to crack during sizing. The sized bearings are immersed in oil under vacuum. The oil impregnates the porosity in the bearings by capillary action. Small pore size is desirable for the most effective capillary action. The inventive bearings develop smaller pores than the standard bronze bearings and consequently give higher oil efficiency. The latter is a measure of the percentage of total porosity in the bearing that is filled with oil. The high radial crush strengths obtained during sintering of the inventive bearings are retained in the sized and oiled bearings. This is an advantage in applications where the bearing loads are high.

The oiled bearings are tested in a bearing tester. The bearing is mounted on a steel shaft, and a load is applied to the shaft. The shaft is rotated at a high speed (rpm). The friction between the shaft and the bearing causes the bearing to heat up. As the bearing gets warmer, the oil goes to the mating surface and provides lubrication. The better the lubrication, the lower the bearing temperature and the lower the wear on the bearing. The temperature peaks early in the bearing test and eventually reaches a steady state level. Lower peak and steady state temperatures are desirable to maintain the integrity of the oil and improve the life of the motor. The bearings of this invention showed lower coefficient of friction and lower peak and steady state temperatures than the standard bronze and diluted bronze bearings. This is attributed to the fine pores and the superior capillary action they provide. All of these observations are confirmed by the Examples 1 and 2 that follow.

Another embodiment of the present invention is to use a copper coated iron powder and adding sufficient tin powder to it to convert the copper into 90:10 bronze during sintering of the parts. Methods to coat the iron powder with copper are known, and include precipitation, electrolysis, etc. Similar proportions of iron, copper and tin in the final powders can be used as described above. Bearings made from these powders are also superior to the premixed bronze and diluted bronze bearings. This will become clear from the Example 3 below.

EXAMPLES

The present invention is illustrated by the following examples. These are merely illustrative and should not be construed as limiting the scope of the invention.

Example 1

70 parts −60 mesh sponge iron powder, 30.3 parts cuprous oxide powder (D₅₀=21 μm) and 3 parts tin powder (D₅₀=8 μm) were blended in a V-cone blender. The blended powder was thermally treated at a range of temperatures from 1300° F. to 1500° F. in hydrogen atmosphere. The milled cake was sieved on a 60 mesh screen. During the thermal treatment, the cuprous oxide was reduced to copper and the resultant powders, labeled A, B and C, contained 70 wt. % iron, 27 wt. % copper and 3 wt. % tin. The copper and tin alloyed with each other and formed bronze.

50 parts −60 mesh sponge iron powder, 50.6 parts fine cuprous oxide powder and 5 parts tin powder were similarly processed. The thermal treatment was carried out at 1300° F. in hydrogen. The resultant powder, labeled D, had a composition of 50 wt. % iron, 45 wt. % copper and 5 wt. % tin.

80 parts −60 mesh sponge iron powder, 20.2 parts fine cuprous oxide powder and 2 parts tin powder were similarly processed. The thermal treatment was carried out at 1300° F. in hydrogen. The resultant powder, labeled E, had a composition of 80 wt. % iron, 18 wt. % copper and 2 wt. % tin.

Table-1 shows the processing parameters for the five powders of this invention. TABLE 1 Thermal Treatment Temperatures Temperature Powder (° F.) A 1300 B 1400 C 1500 D 1300 E 1300

FIGS. 1, 2 and 3 show photomicrographs of Powders A, D and E, respectively. They show that most of the iron and bronze particles are diffusion bonded to each other. This prevents segregation and gives a more uniform powder. As will become clear from the data presented below, a thermal treatment at 1300° F. is sufficient to produce a good quality bronze for the bearings.

Two grades of diluted bronze powders were made by blending 70 parts −60 mesh sponge iron powder and 30 parts −150 mesh premixed bronze powder (labeled F), and 50 parts −60 mesh sponge iron powder and 50 parts −150 mesh premixed bronze powder (labeled G).

The powder properties were measured. A standard premixed bronze powder (90 wt. % copper+10 wt. % tin) was used for comparison. The data are shown in Table-2 below. TABLE 2 Properties of Unlubricated Powders Apparent +60 +80 +100 +150 +200 +325 −325 Powder Density g/cm³ Flow s/50 g mesh mesh mesh mesh mesh mesh Mesh A 2.24 36 0.03 4.75 8.24 14.97 24.35 24.35 23.31 B 2.09 41 0.21 14.84 12.89 21.55 19.32 19.04 7.09 C 2.12 43 1.29 29.55 17.76 16.39 16.82 11.10 7.09 D 2.05 38 0.25 11.36 9.92 17.60 17.55 21.25 22.07 E 2.22 36 0.0 0.40 4.50 20.0 29.90 22.90 22.30 F — — Trace 2.10 4.9 13.7 20.1 29.3 29.9 G — — None 1.4 3.4 9.5 15.9 30.9 28.9 Std — — None None Trace 0.1 7.0 32.2 60.7

The powders were blended with 0.50 wt. % zinc stearate and 0.25 wt. % lithium stearate as lubricants. The properties of the lubricated powder are shown in Table-3. TABLE 3 Properties of Lubricated Powders Apparent Density Flow Powder (g/cm³) (s/50 g) A 2.62 33 B 2.34 39 C 2.33 42 D 3.11 27 E 2.49 34 F 3.02 27 G 3.11 28 Std 3.14 34

The powders were pressed into standard transverse rupture strength bars to 6.3 g/cm³ density and the green strength was measured—see Table-4. TABLE 4 Green Strength Green Strength Powder (lb/in²) A 1,075 B 1,483 C 1,826 D 1170 E 1,200 F 1,270 G 980 Std 770

All five powders made by the process of this invention (powders A through E) had green strength higher than the standard premixed bronze powder and at least as good as the diluted bronze powders.

The powders were pressed into bearings with nominally 0.75 inch inside diameter X 1.0 inch outside diameter X 0.75 inch length. They were preheated at 1000° F. to burn off the lubricant and sintered at temperatures ranging from 1400° F. to 1560° F. in a belt conveyor furnace under a reducing atmosphere composed of 75% hydrogen and 25% nitrogen.

FIGS. 4 through 6 show the photomicrographs of the sintered bearings made from powders A, D and E of this invention. FIGS. 7 and 8 show the photomicrographs of the bearings made from the diluted bronze powders F and G respectively. FIG. 9 shows the photomicrograph of the sintered bearing made from the premixed bronze powder. The bearings of the present invention clearly show a finer pore structure (dark areas) than the conventional premixed and diluted bronze bearings.

FIG. 10 compares the DC of sintered bearings of this invention with the standard premixed bronze bearings and the diluted bronze bearings as a function of the sintering temperature. The data shows that the bearings made from the powders of this invention undergo consistent and low DC during sintering. This is desirable because it provides predictability in terms of die sizes required for the sizing operation and also minimizes deformation necessary during sizing. The DC is also stable over the temperature range normally used in the industry. This is very desirable in that it provides some flexibility in the sintering temperature needed to achieve the desired sintered properties.

FIG. 11 compares the RCS of sintered bearings of this invention with the standard premixed bronze bearings and diluted bronze bearings as a function of the sintering temperature. The bearings of this invention have higher RCS than the premixed and diluted bronze bearings over the entire temperature range. One major advantage of the powders of this invention is that the bearings can be sintered at 1400° F., and still achieve a higher RCS than bearings made from the standard premixed bronze and the diluted bronze powders sintered at higher temperatures. Lower sintering temperature is beneficial from the energy savings standpoint, and adds to the already lower cost of the powder resulting from replacement of some of the bronze powder by iron powder.

The bearings sintered at 1400 and 1500° F. were immersed in oil under vacuum. The oil penetrated the porosity by capillary action. Table-5 shows the oil content and oil efficiency (% pores filled with oil) data on these bearings. TABLE-5 Oil Content and Oil Efficiency Data on Sintered Bearings Sintered Impregnated Submerged Dry Oiled Oil Oil Weight Weight Weight Density Density Content Efficiency Powder (g) (g) (g) (g/cm3) (g/cm3) (cc) (%) Sintered at 1400° F. A 26.1779 26.9124 22.8280 6.40 6.57 20.3 95 B 23.2039 23.9036 20.2274 6.30 6.49 21.5 96 C 23.0651 23.7739 20.1057 6.27 6.47 21.9 96 D 26.0109 26.8336 22.7430 6.35 6.55 22.8 96 E 27.0404 27.71070 23.5921 6.54 6.70 17.8 94.9 Sintered at 1500° F. A 26.2759 27.0258 22.9000 6.35 6.54 20.6 94 B 23.1570 23.8613 20.1872 6.29 6.48 21.7 96 C 23.3897 24.0669 20.3948 6.36 6.54 20.9 96 D 26.1020 26.9199 22.8225 6.36 6.56 22.6 96 E 27.1615 27.5923 23.6521 6.30 6.49 20.8 96 F 27.1784 28.1557 23.6311 5.99 6.21 24.4 97 G 27.2645 28.2389 23.8145 6.15 6.37 24.9 98 Std 27.2026 28.2244 23.9014 6.28 6.52 26.8 94

The bearings made from the powders of this invention had oil efficiencies (94-96%) comparable to the standard premixed bronze bearings (94%) and the diluted bronze bearings (97-98%). Oil efficiency is measured as a percentage of porosity present in the bearings that is filled with oil.

The sintered bearings made from powder A (70 wt. % iron, 27 wt. % copper, 3 wt. % tin), powder D (50 wt. % iron, 45 wt. % copper, 5 wt. % tin), powder E (80 wt. % iron, 18 wt. % copper, 2 wt. % tin), powder F (70 wt. % iron, 30 wt. % premixed bronze), powder G (50 wt. % iron, 50 wt. % premixed bronze), and the standard premixed bronze were sized and filled with oil as indicated above. The premixed bronze and diluted bronze bearings sintered at higher temperatures were used here in order to meet the DC and RCS requirements. Table-6 shows the oil content and oil efficiency of these bearings. TABLE-6 Oil Content and Oil Efficiency Data on Sized Bearings Sized Impregnated Submerged Dry Oiled Oil Oil Weight Weight Weight Density Density Content Efficiency Powder (g) (g) (g) (g/cm3) (g/cm3) (cc) (%) Sintered at 1400° F. A 26.1600 26.8544 22.8682 6.55 6.72 18.9 96 D 26.0600 26.8798 22.8595 6.47 6.67 22.12 94 E 27.0404 27.7155 23.5937 6.55 6.71 17.8 95 Sintered at 1500° F. Std 27.1400 28.0742 23.9290 6.53 6.76 24.4 93 Sintered at 1560° F. F 27.178 27.9019 23.6735 6.41 6.58 18.6 88 G 27.1299 27.9354 23.7185 6.41 6.60 20.8 91

Bearings that met the industry specifications, including RCS, were selected for bearing test performance. They were tested on a Falex F 1505 bearing tester. The bearings were evaluated with a shaft speed of 1750 rpm and 85 lbs load to attain PV values of around 50,000. The bearings reach a peak temperature and then drop down to a stable steady state operating temperature. The coefficient of friction was calculated from the formula ((Torque reading from load cell−in.lb/Shaft radius−in.)/Load suspended on bearing−lb.)). Oil loss was measured after the bearing test was completed. Table 7 shows the properties of the bearings tested. TABLE 7 Bearing Test Data Steady Peak State Coeff. Oil PV Temp. Temp. PVμ of Loss Powder (psi · ft/min) (° F.) (° F.) (psi · ft/min) Friction (%) A 53,372 198 196 607 0.011 9.8 D 54,196 190 188 815 0.015 8.4 E 53,300 158 158 388 0.007 1.3 F 52,905 212 198 908 0.017 9.3 G 55,029 202 192 1,122 0.021 14.3 Std 52,557 232 199 999 0.019 14.2

The bearings made from the powders of this invention (A, D and E) had lower coefficients of friction than the bearings made from the standard premixed bronze (Std.) and the diluted bronze powders (F and G). This is due to the smaller pores in the bearings, which provide more efficient transport of oil to the mating surface between the bearing and the shaft. The lower coefficient of friction leads to lower peak and steady state operating temperatures in the bearing. This translates into lower oil loss, and is desirable from the standpoint of the oil integrity and the long term performance of the bearing.

Example 2

In this example the sponge iron powder in Example 1 above was replaced with an atomized iron powder. The rest of the processing was similar.

70 parts −60 mesh atomized iron powder, 30.3 parts cuprous oxide powder (D₅₀=21 μm) and 3 parts tin powder (D₅₀=8 μm) were blended in a V-cone blender. The blended powder was thermally treated at 1300° F. in hydrogen atmosphere. The milled cake was sieved on a 60 mesh screen. The resultant powder was labeled H.

50 parts −60 mesh atomized iron powder, 50.6 parts fine cuprous oxide powder and 5 parts tin powder were similarly processed. The thermal treatment was carried out at 1300° F. in hydrogen. The resultant powder was labeled I.

Two grades of diluted bronze powders were made by blending 70 parts −60 mesh atomized iron powder and 30 parts −150 mesh premixed bronze powder (labeled J), and 50 parts −60 mesh atomized iron powder and 50 parts −150 mesh premixed bronze powder (labeled K).

The microstructures of these powders were similar to the corresponding powders made using the sponge iron powder as in Example 1, and are not shown here. Table-8 shows the properties of these powders. TABLE 8 Properties of Unlubricated Powders Apparent Density Flow +60 +80 +100 +150 +200 +325 −325 Powder (g/cm³) (s/50 g) (mesh) (mesh) (mesh) (mesh) (mesh) (mesh) (mesh) H 2.18 37 0.3 17.0 14.4 20.8 18.9 17.2 11.4 I 2.00 42 0.4 19.9 13.2 17.9 17.1 17.7 13.8 J — — None 0.7 6.3 14.4 18.9 30.0 29.7 K — — None 0.4 4.6 10.0 15.1 31.3 38.6

Properties of the powders blended with 0.5 wt. % zinc stearate and 0.25 wt. % lithium stearate are shown in Table-9. TABLE 9 Properties of Lubricated Powders Apparent Density Flow Powder (g/cm³) (s/50 g) H 2.45 37 I 2.29 39 J 3.31 27 K 3.35 27

The powders were pressed into standard transverse rupture strength bars to a nominal 6.3 g/cm³ density and the green strength was measured—see Table-10. TABLE 10 Green Strength Green Strength Powder (lb/in²) H 745 I 1107 J 564 K 575

The powders of this invention (H and I) have superior green strength compared to the diluted bronze powders made using the same atomized iron powder, and the standard premixed bronze of Example I (Table-4).

The powders were pressed into bearings with nominally 0.75 inch inside diameter X 1.0 inch outside diameter X 0.75 inch length. They were preheated at 1000° F. to burn off the lubricant and sintered at a temperature in the 1400° F. to 1560° F. range, in the belt conveyor furnace under a reducing atmosphere composed of 75% hydrogen and 25% nitrogen.

Table-11 shows the DC and RCS of sintered bearings of this invention and the corresponding diluted bronze bearings. Like the previous bearings of this invention using the sponge iron powders (A through E), the bearings made using the atomized iron powder (H and I) have lower DC and higher RCS values than the corresponding diluted bronze bearings (J and K). TABLE 11 DC and RCS of Sintered Bearings Sintering Dim. Radial Crush Temperature Change Strength Powder (° F.) (%) (lb/in²) H 1530 −0.05 36,050 I 1400 −0.38 32,300 J 1530 0.60 33,800 K 1530 0.17 36,200

FIGS. 12 through 15 show the photomicrographs of the sintered bearings. The bearings of this invention (powders H and I) show finer pore structures than the corresponding diluted bronze bearings (powders J and K).

The sintered bearings were immersed in oil under vacuum. Table-12 shows the oil content and oil efficiency data on these bearings. TABLE 12 Oil Content and Oil Efficiency Data on Sintered Bearings Dry Oiled Oil Oil Density Density Content Efficiency Powder (g/cm³) (g/cm³) (cm³) (%) H 6.28 6.48 22.5 98 I 6.37 6.57 22.0 94 J 6.20 6.40 22.4 94 K 6.34 6.54 22.2 93

The bearings made from the powders of this invention had oil efficiencies at least as good as or better than the diluted bronze bearings and the standard premixed bronze bearings of Example-1.

The sintered bearings were sized and filled with oil as described previously. Table-13 shows the oil content and oil efficiency of these bearings. The sized bearings of this invention were at least as good as or better than the diluted bronze bearings and the standard premixed bronze bearings of Example-1. TABLE 13 Oil Content and Oil Efficiency Data on Sized Bearings Dry Oiled Oil Oil Density Density Content Efficiency Powder (g/cm³) (g/cm³) (cm³) (%) H 6.45 6.64 20 98 I 6.45 6.63 20 94 J 6.46 6.64 19 96 K 6.44 6.64 21 97

The bearings were performance tested as described in Example-1. Table-14 shows the bearing performance data. TABLE 14 Bearing Test Data Steady Peak State Coeff. Oil PV Temp. Temp. PVμ of Loss Powder (psi · ft/min) (° F.) (° F.) (psi · ft/min) Friction (%) H 53,385 181 181 711 0.014 3.2 I 53,806 155 155 704 0.013 5.9 J 52,905 212 198 908 0.017 7.4 K 55,029 203 192 1,122 0.017 9.4

The bearings made from the powders of this invention had lower coefficients of friction than the bearings made from the diluted bronze powders and the standard premixed bronze powder (Table-7). The lower coefficients of friction resulted in lower peak and steady state operating temperatures in the bearing. This, in turn, translated into lower oil loss, which would be expected to give better oil integrity and long term performance of the bearings.

The bearing performance data in Tables-7 and 14 shows that the sponge iron and atomized iron powders are equivalent in practicing this invention.

Example-3

In this example, a commercially available copper coated iron powder was used to produce bearings. FIG. 18 shows a photomicrograph of this powder. The bearings made from this powder were compared with the bearings made from powder E, having a similar composition, and the standard premixed bronze powder discussed in Example-1. The copper coated iron powder contained 82% iron (core) and 18% copper (coating). 98 parts of this powder were blended with 2 parts of fine tin powder, 0.50 parts of zinc stearate and 0.25 parts of lithium stearate. The powder was labeled L.

Table-15 shows the properties of this powder along with powder E and the standard premixed bronze powder. The powder properties are similar to powder E. TABLE 15 Properties of Lubricated Powders Apparent Density Flow Powder (g/cm³) (s/50 g) L 2.20 41 E 2.49 34 Std. 3.14 34

The powders were pressed into standard transverse rupture strength bars to a nominal 6.3 g/cm³ density and the green strength was measured—see Table-16. TABLE 16 Green Strength Green Strength Powder (lb/in²) L 2,250 E 1,200 Std. 770

The powder L has green strength superior to powder E and the standard premixed bronze powder of Example 1.

The powder was pressed into bearings with nominally 0.75 inch inside diameter X 1.0 inch outside diameter X 0.75 inch length. They were preheated at 1000° F. to burn off the lubricant and sintered at 1400° F. in the belt conveyor furnace under a reducing atmosphere composed of 75% hydrogen and 25% nitrogen.

Table-17 shows the DC and RCS of the sintered bearings, comparing them to bearings made from powder E and the standard premixed bronze powder. The bearings made from powder L have a DC similar to those made from powder E. Powder L also gives an RCS similar to powder E, and much higher than the premixed bronze powder. The standard premixed bronze bearings were sintered at a higher temperature (1500° F.) to obtain the desired RCS. TABLE 17 DC and RCS of Sintered Bearings Sintering Radial Crush Temperature Dim. Change Strength Powder (° F.) (%) (lb/in²) L 1400 −0.02 42,200 E 1400 0.10 40,300 Std. 1500 0.01 31,650

FIG. 19 shows the photomicrograph of a sintered bearing. The bearing shows a similar pore size as the bearing made from powder E (FIG. 6), but a finer pore size than the bearing made from the premixed bronze powder (FIG. 9).

The sintered bearings were immersed in oil under vacuum. Table-18 shows the oil content and oil efficiency data on these bearings. TABLE 18 Oil Content and Oil Efficiency Data on Sintered Bearings Dry Oiled Oil Oil Density Density Content Efficiency Powder (g/cm³) (g/cm³) (cm³) (%) L 6.35 6.53 19.9 94 E 6.33 6.51 20.4 95 Std. 6.28 6.52 25.6 94

The bearings made from the powders of this invention had oil efficiencies similar to powder E and the standard premixed bronze bearings of Example-1.

The sintered bearings were sized and filled with oil as described previously. Table-19 shows the oil content and oil efficiency of these bearings. The sized bearings made from powder L were similar to bearings made from powder E and the standard premixed bronze bearings of Example-1. TABLE 19 Oil Content and Oil Efficiency Data on Sized Bearings Dry Oiled Oil Oil Density Density Content Efficiency Powder (g/cm³) (g/cm³) (cm³) (%) L 6.52 6.68 18 93 E 6.55 6.72 18 95 Std. 6.52 6.75 24 95

The bearings were tested as described in Example-1. Table-20 shows the bearing performance data. TABLE 20 Bearing Test Data Steady Peak State Coeff. Oil PV Temp. Temp. PVμ of Loss Powder (psi · ft/min) (° F.) (° F.) (psi · ft/min) Friction (%) L 53,076 175 175 635 0.012 3.6 E 53,300 158 158 388 0.007 1.3 Std 52,557 232 199 999 0.019 14.2

The bearings made from the powder L had a lower coefficient of friction than the bearings made from the standard premixed bronze powder. The lower coefficient of friction resulted in lower peak and steady state operating temperatures in the bearings. This, in turn, translated into lower oil loss, which would be expected to give better oil integrity and long term performance of the bearings. The coefficient of friction, the peak and steady state operating temperatures, and the oil loss were similar to the bearings made from powder E.

CONCLUSION

The bearings made from the powders of this invention (A, B, C, D, E, H, I and L) showed superior performance in the bearing tests than the bearings made from the standard premixed bronze powder (Std.) and the diluted bronze bearings (F, G, J and K). They had lower coefficients of friction, which resulted in lower peak and steady state operating temperatures, and less oil loss. The latter is a sign of better oil integrity and excellent bearing performance over a long period of time.

While the invention has been described with reference to specific embodiments thereof, it should be understood that the invention is capable of further modifications and that this application is intended to cover any and all variations, uses, or adaptations of the invention which follow the general principles of the invention. All such alternatives, modifications and equivalents that may become obvious to those of ordinary skill in the art upon reading the present disclosure are included within the spirit and scope of the invention as reflected in the appended claims. 

1. A process for making a bronze powder comprising the following steps: a. blending an iron powder with a cuprous oxide powder and a tin powder to form a blended powder, b. thermally treating the blended powder in a reducing atmosphere at a temperature between about 600° F. and about 1700° F. to form a sintered cake, and c. milling the sintered cake to form the bronze powder.
 2. The process of claim 1 which further comprises screening the bronze powder to a desired particle size fraction.
 3. The process of claim 1, wherein the iron powder is sponge, atomized, or electrolytic iron powder.
 4. The process of claim 3, wherein the iron powder is sponge iron powder.
 5. The process of claim 1, wherein the iron powder has a particle size between about −325 mesh (<44 μm) and about −60 mesh (<250 μm).
 6. The process of claim 5, wherein the iron powder has a particle size of approximately −150 mesh (<106 μm).
 7. The process of claim 1, wherein the cuprous oxide powder has a median particle size (D₅₀) below about 50 μm.
 8. The process of claim 7, wherein the cuprous oxide powder has a median particle size (D₅₀) of about 10-25 μm.
 9. The process of claim 1, wherein the tin powder has a median particle size (D₅₀) below about 20 μm.
 10. The process of claim 9, wherein the tin powder has a median particle size (D₅₀) of about 3-12 μm.
 11. The process of claim 1, wherein the thermal treatment temperature is between about 800° F. to about 1600° F.
 12. The process of claim 11, wherein the thermal treatment temperature is about 1300° F.
 13. The process of claim 1, wherein the reducing atmosphere is pure hydrogen or a nitrogen/hydrogen mixture.
 14. A bronze powder made by the process of claim
 1. 15. The powder of claim 14, wherein the bronze powder has a particle size below about −20 mesh.
 16. The powder of claim 15, wherein the bronze powder has a particle size of about −60 mesh.
 17. The powder of claim 14, wherein the bronze powder has a copper content of about 10-70 wt. %.
 18. The powder of claim 17, wherein the bronze powder has a copper content of about 20-50 wt. %.
 19. The powder of claim 14, wherein the bronze powder has a tin content of about 1-7 wt. %.
 20. The powder of claim 19, wherein the bronze powder has a tin content of about 2-5 wt. %.
 21. The powder of claim 14, wherein the bronze powder is blended with 0.25 wt. % to 1.00 wt. % powdered lubricant.
 22. The powder of claim 21, wherein the lubricant is a stearate or wax.
 23. A sintered part comprising a powder made by the process of claim
 1. 24. The sintered part of claim 23, wherein the part is a bearing.
 25. The sintered part of claim 24, wherein the bearing is a oil impregnated self lubricating bearing.
 26. A composite powder made by mixing the powder of claim 1 with other alloying elements, plastics, solid lubricants, ceramics, or the like.
 27. An article made from the powder of claim
 26. 28. A sintered part having 10 to 70 wt. % copper, 1 to 7 wt. % tin and 23 to 89 wt. % iron, wherein a substantial portion of pores present in the sintered part have dimensions less than 100 μm, and wherein the sintered part has a radial crush strength higher than 30,000 psi at a nominal sintered density of 6.3 g/cm³. 